Demand for secondary batteries has markedly increased with the development and increasing usage of mobile devices, and particularly, secondary batteries having high energy density, high operating voltages, easy-to-store characteristics, and relatively long lifespans are widely used as power sources in various electronic products such as mobile devices.
Generally, secondary batteries such as lithium secondary batteries are formed by disposing an electrode assembly and an electrolyte in a battery case and sealing the battery case. Secondary batteries may be classified as cylindrical, prism, and pouch type batteries according to the shapes thereof, and lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries according to the types of electrolytes used therein. Demand for thin prism or pouch type batteries has increased as the size of mobile devices has decreased, and particularly, pouch type batteries are noteworthy due to being relatively lightweight.
Electrode assemblies accommodated in battery cases may be classified into jelly-roll (wound), stacked, and stacked/folded (combination) types according to the structures thereof. Generally, jelly-roll type electrode assemblies may be manufactured by coating metal foil used as a current collector with an electrode active material, pressing the metal foil, cutting the metal foil into a band having a desired width and length, separating positive and negative electrodes using a separation film, and winding the separation film in a spiral shape. Stacked type electrode assemblies may be manufactured by vertically stacking a negative electrode, a separator, and a positive electrode. Stacked/folded type electrode assemblies may be manufactured by placing respective electrode stacks having a single electrode or a structure of a negative electrode/a separator/a positive electrode on a long sheet of separation film, and folding the separation film.
However, in the related art, electrode assemblies are usually manufactured by stacking unit cells or electrodes having the same size and thus have limited shapes. Therefore, there are many limitations in terms of battery designs. To address such limitations, methods of manufacturing batteries having stepped portions by using electrodes or unit cells having different sizes have been introduced. However, since such batteries having stepped portions are manufactured by cutting positive electrode plates and negative electrode plates into different sizes to form unit cells having different sizes and stacking the unit cells, the thicknesses of layers of the batteries are limited to multiples of the thicknesses of the unit cells, and thus the degree of design freedom of the batteries is relatively low.
Furthermore, the above-mentioned technique of the related art only discloses a simple idea of changing the design of a battery by cutting positive and negative electrodes into desired sizes and stacking the positive and negative electrodes. That is, a specific method of manufacturing batteries having practically useful battery characteristics is not disclosed. For example, although unit cells of a battery having a stepped portion have no individual operational errors, the unit cells may have overall operation errors, according to the stacked structure of the unit cells, or the electrical capacity of the battery having a stepped portion may be very low as compared with the capacity of a battery having the same volume. In addition, as charging and discharging cycles are repeated, interfaces between layers of the battery having a stepped portion may swell excessively, lowering the lifespan of the battery and making it difficult to use the battery practically. However, in the related art, batteries having stepped portions are not designed with the prevention of such problems in mind.
Therefore, high-capacity, durable electrode assemblies that can be variously designed according to the shapes of devices in which the electrode assemblies are to be used, and the development of batteries using such electrode assemblies is needed.